The present invention relates to a frequency deviation detecting apparatus and a frequency deviation detecting method for detecting frequency deviation of a DTMF signal. This invention relates more particularly to a frequency deviation detecting apparatus and a frequency deviation detecting method for detecting frequency deviation of a DTMF signal with high precision.
As a signaling used for a push-button telephone set, there are DTMF (Dual Tone Multi Frequency) recommended by ITU (International Telecommunication Union), Q22, Q23, and Q24. In the DTMF, one frequency is selected out of the four frequencies in a low frequency group (697, 770, 852, and 941 Hz) and four frequencies in a high frequency group (1209, 1336, 1447, and 1633 Hz) respectively and the selected frequencies are added to generate a DTMF signal. FIG. 13 is a view showing symbols represented by combinations of the low frequency group and high frequency group based on the conventional technology. As shown in the figure, the DTMF has 16 symbols (shown in the row of symbol in this figure) based on combinations of the low frequency group and the high frequency group of a DTMF signal.
When a DTMF signal is to be received, frequency deviation, twist, and a signal level or the like of the signal are checked, and if the signal satisfies certain standards, it is recognized as a DTMF signal. Herein, frequency deviation is a value indicating by what percent a frequency mainly included in a received signal is deviated as compared to the prescribed low frequency group and high frequency group. If the frequency deviation is below 1.8%, the signal is received as a DTMF signal, but if the frequency deviation is more than 3.0%, the signal is not received as a DTMF signal.
Furthermore, there are restrictions such that a signal with a minimum duration (40 msec), but not less than 24 msec, is not received. For example, when the frequency of a received signal is to be analyzed for detecting frequency deviation, it is required to set analytical blocks for analysis so that at least one analytical block is ensured within the minimum lifetime of a DTMF signal, considering deviation between start of the received signal and start of the analytical blocks because there is no synchronization between the received signal and analysis. Namely, it is required to maintain a certain throughput of analysis within the restrictions of the specifications of DTMF. Conventionally, analysis is carried out in an analytical block with about 105 samples (13 msec) of a sound signal at an 8-kHz sampling rate.
As a conventional type of frequency deviation detecting apparatus for detecting frequency deviation of a DTMF signal, there is a DTMF receiver which analyzes frequencies of a prescribed number of samples of received signals, for example, around 105 samples at 8 kHz sampling rate on eight prescribed frequencies (four frequencies in the low frequency group and four frequencies in the high frequency group) to extract the frequency components, and detects the frequency deviation from each signal strength of the frequency components.
FIG. 14 is a block diagram showing general configuration of the conventional DTMF receiver. The conventional DTMF receiver comprises a frequency analyzer 1 for analyzing frequency components of a received signal; and a DTMF signal determining unit 3 for receiving the results of analysis regarding the frequency components from the frequency analyzer 1, checking the frequency deviation, the twist, and the signal level or the like to determine whether the signal is a DTMF signal or not. The DTMF signal determining unit 3 outputs, when it is determined that the signal is a DTMF signal, information according to any combination of the low frequency group and high frequency group to a 5-bit bus 2.
The frequency analyzer 1 comprises a frequency detector 11 for detecting strength of a 697 Hz frequency component, a frequency detector 12 for detecting strength of a 770 Hz frequency component, a frequency detector 13 for detecting strength of a 852 Hz frequency component, a frequency detector 14 for detecting strength of a 941 Hz frequency component, a frequency detector 15 for detecting strength of a 1209 Hz frequency component, a frequency detector 16 for detecting strength of a 1336 Hz frequency component, a frequency detector 17 for detecting strength of a 1447 Hz frequency component, and a frequency detector 18 for detecting strength of a 1633 Hz frequency component.
In this DTMF receiver, each of the frequency detectors 11-18 provided in the frequency analyzer 1 receives a signal, subjects the signal to DFT (Discrete Fourier Transform) for each of the prescribed frequencies (697, 770, 852, 941, 1209, 1336, 1447, and 1633 Hz) with a method that uses the Goertzel algorithm, and detects strength of each frequency component. The Goertzel algorithm is an algorithm which performs DFT similarly to a FFT (Fast Fourier Transform) algorithm, and is advantageously used when only a limited number of frequency components are to be detected. Generally in DFT, longer analytical blocks lead to higher frequency precision.
The DTMF signal determining unit 3 receives results of analysis of the frequency components of the received signal, namely receives each strength of the frequency components detected by the frequency detectors 11-18 for comparison. FIG. 15 is a graph showing an example of a conventional type of received signal. In the example of FIG. 15, 697 Hz represents the highest strength of the low frequency group (LG), while 1336 Hz represents the highest strength of the high frequency group (HG). Therefore, it can be estimated that the received signal is a DTMF signal based on a combination of 697 Hz and 1336 Hz. Frequency deviation, signal strength, and twist or the like of the 697 Hz and 1336 Hz frequency components are checked, and it is determined whether the signal is a DTMF signal or not according to the specifications. Herein, frequency deviation can be computed by using the fact that the strength of frequency components in prescribed frequencies is reduced if frequency deviation is larger, in other words, if a peak of the strength of the frequency components is more deviated from the prescribed frequencies.
FIG. 16 is an explanatory view showing the operation of the conventional type of DTMF receiver. In the operation of the conventional type of DTMF receiver, the frequency analyzer 1 performs frequency analyses of a received signal in each analytical block having 105 samples (about 13 msec) at 8 kHz sampling rate continuously, as in the analytical block 0 to the analytical block 4. Namely, the analytical blocks are linked to each other such that the analytical block 0 is checked and then the analytical block 1 is checked. The DTMF signal determining unit 3 receives the result of analysis of the frequency components from the frequency analyzer 1 for 105 samples, checks the frequency deviation, the twist, and the signal level or the like, and determines whether the signal is a DTMF signal or not.
With the above mentioned conventional technology, however, analysis is carried out in a short analytical block, for example with 105 samples (about 13 msec) at 8 kHz sampling rate, and frequency deviation is estimated only from the strength of the frequency components at prescribed frequencies. Therefore sufficient precision in detecting prescribed frequency deviation of 1.8% can not be obtained and there are some cases where appropriate frequency deviation can not be detected. Furthermore, analysis is carried out in continuous analytical blocks, therefore when analysis is to be performed in a sufficiently long analytical block, there occurs a case where at least one analytical block can not be ensured within the minimum lifetime of a DTMF signal.
It is an object of the present invention to obtain, for the purpose of solving the problems described above, a frequency deviation detecting apparatus which can detect appropriate frequency deviation by ensuring sufficient precision to detect prescribed frequency deviation while at least one analytical block is ensured within the minimum lifetime of a DTMF signal.
In the present invention, the received signal is analyzed for frequency components around the prescribed frequencies in a plurality of analytical blocks each having a length that ensures the precision required for detecting the prescribed frequency deviation, and the analytical blocks are displaced from and overlaid with each other so that at least one analytical block is ensured within the minimum lifetime of a DTMF signal.
Further, in the present invention, the received signal is analyzed for frequency components of a plurality of prescribed frequencies, which combination of a high frequency group with a low frequency group forming the prescribed frequency is mainly included in a received signal is detected, and only components of frequencies around the detected high frequency group and low frequency group are analyzed for detection of frequency deviation.
Further, in the present invention, the displacement between the analytical blocks for detection of frequency deviation is around 105 samples at 8 kHz sampling rate, and a length of the analytical block is around 150 samples at 8 kHz sampling rate.
Further, in the present invention, the received signal is analyzed for frequency components of a plurality of prescribed frequencies in an analytical block with around 60 samples at 8 kHz sampling rate that is longer enough to detect which frequency component of the prescribed frequency is included in the signal and is shorter than the analytical block based on the conventional technology.
In the method of the present invention, a received signal is analyzed for components of frequencies around a prescribed frequency in a plurality of analytical blocks each having a length that ensures the precision required for detecting the prescribed frequency deviation, and the plurality of analytical blocks are displaced from and overlaid with each another so that at least one analytical block is ensured within the minimum lifetime of a DTMF signal.
Further, in the present invention, the received signal is analyzed for frequency components of a plurality of prescribed frequencies, which combination of a high frequency group with a low frequency group forming the prescribed frequency is mainly included in the received signal is detected, and only components of frequencies around the detected high frequency group and low frequency group are analyzed for detection of frequency deviation.